This invention relates to a digital-to-analog converter for converting a digital input signal into an analog output signal, comprising
an input terminal for receiving the digital input signal, PA1 an output terminal for supplying the analog output signal, PA1 a current source circuit having N current sources and adapted to generate N currents in a well-defined ratio at N outputs, PA1 a combination circuit for composing an analog signal, as a function of the digital input signal, from the currents available at the outputs of the current source circuit, said combination circuit having N inputs coupled to the N outputs of the current source circuit, another input coupled to the input terminal for receiving the digital input signal and an output coupled to the output terminal.
A digital-to-analog converter of this type is known from U.S. Pat. No. 4,703,310 (10/27/87).
This type of digital-to-analog converter uses the dynamic permutation principle known from U.S. Pat. No. 3,982,172 and U.S. Pat. No. 4,125,803 which Patents are hereby incorporated by reference. According to this principle currents having an accurate mutual amplitude ratio are generated in the current source circuit by coupling substantially equal currents to the outputs in accordance with a cyclic permutation. The known current source circuit comprises a permutation circuit for this purpose. Due to the cyclic permutation the relative deviation of each of the originally generated currents with respect to a mean value appears equally often per cycle in each of the output currents. Consequently, each output current has a D.C. component having a very accurate amplitude ratio with respect to the mean value of the originally generated currents and hence with respect to each D.C. component of the other output currents. The mutual deviations between the originally generated currents appear as A.C. components in these output currents, which A.C. components may or may not be disturbing, dependent on the permutation frequency with respect to the highest signal frequency and dependent on the mutual amplitude ratio of the originally generated currents.
In those cases in which these A.C. components may be disturbing, they may be filtered out by adding an averaging circuit to each output of the current source circuit. This make it possible to perform a digital-to-analog conversion without any problems by means of the combination circuit and with the aid of the thus generated direct currents. The aforementioned amplitude ratio is each time a factor of two. The currents to be switched are then directly switched on by an associated bit of the digital signal so as to generate the analog signal corresponding to the digital signal.
To obtain the currents in a well-defined ratio, the current source circuit uses shift registers whose number increases as the number of currents to be generated in the current source circuit increases. This results in a large power dissipation in the known D/A converter.